Wire enamel formulation with internal lubricant

ABSTRACT

PCT No. PCT/EP96/01723 Sec. 371 Date Jan. 13, 1998 Sec. 102(e) Date Jan. 13, 1998 PCT Filed Apr. 25, 1996 PCT Pub. No. WO96/34399 PCT Pub. Date Oct. 31, 1996The present invention relates to a wire enamel formulation comprising components which are known per se and an internal lubricant consisting of a polyethylene wax, preferably having a molecular mass of from 3000 to 6000 g/mol, and a wetting agent, preferably fatty alcohol ethoxylate.

The present invention relates to a wire enamel formulation obtainingcomponents known per se with an internal lubricant.

Enamelled copper wires are coated with a lubricant in order to improvetheir processing properties. Conventional lubricants consist of a 0.5 to2% strength solution of paraffins or waxes in a readily volatilesolvent. On application to the wire, the solvent evaporates to leave theparaffin or was film. A disadvantage of this procedure is that thesolvents customarily used may give rise to cracks in the surface of theenamel film.

These and other disadvantages disappear if internal lubricants are used.Such lubricants are added to the enamelling material. After the wireenamel has cured, they are incompatible with it. They migrate to thesurface where they form a layer which possesses improved lubricity.

The problem is that many of these internal lubricants are incompatiblewith the liquid enamelling material and lead to phase separation orinstances of precipitation.

DE 32 37 022A describes a lubricant comprising an aliphatic hydrocarbonmixture as solvent and 1% paraffin wax and also 1% hydrogenatedtriglyceride. The paraffin wax has a melting point of 50-52° C. Thehydrogenated triglyceride is a commercial product with a melting pointof from 47° C. to 50° C. this solution is applied to a wire coated witha polyamideimide. In addition, it is also possible for an internallubricant to be used. The latter is added to the polyamideimide in aconcentration of 1%. The internal lubricant consists of tall oil fattyacid esters. No information is given on the coefficients of frictionwhich are achieved.

EP 00 72 178A describes the modification of wire enamel binders, in thecourse of which a C21 hydrocarbon chain is incorporated into thepolymer. This chain leads to an improved coefficient of friction in theenamelled wires. The document gives no information on the thermalproperties. It is suspected that the softening of the enamel film andthe dielectric loss factor are adversely affected by the introduction ofthe hydrocarbon chain.

EP 0 103 307A describes conventionally applied lubricants which, on thewires in relays, show a reduced gas-escape tendency. This is achieved bythe substitution of the terminal hydrogen in a polypropylene glycol byan organic radical.

EP 0 267 736 describes comparisons between paraffinic and polymericlubricants. In the relay reliability test, the polymeric lubricantsperform considerably better. No information is given on the stability ofthe processed polymer/wire enamel mixtures.

Another document (JP 0 524 7374A) describes how the use of dispersionsof fluorinated waxes in conventional wire enamels leads to animprovement in the lubricity of the wires produced therewith. Suchsystems have a tendency, however, to undergo phase separation.

JP 0 521 7427A describes the use of a polyethylene wax dispersion in apolyamideimide wire enamel. Experience indicates that these systems arenot stable on storage.

From the examples given it is evident that an optimum lubricant must bean internal lubricant. Moreover, the lubricity-improving additive shouldbe a polymeric material, and the formulation should be stable onstorage.

The object of the present invention, accordingly, was to provide a wireenamel formulation obtaining components known per se and an internallubricant, which formulation meets the requirements set cut above.

This object is surprisingly achieved in that the internal lubricantobtains a polyethylene wax, preferably having a molecular means [M_(W) ]of from 3000 to 6000 [g/mol], and a wetting agent, preferably fattyalcohol ethoxylate.

In accordance with the invention it is possible to use wire enamelscontaining a polyesterimide as binder. Such polyesterimide resins areknown and are described, for example, in DE-A 14 45 263 and DE-A 14 95100.

The polyesterimides are prepared in a known manner by esterification ofpolybasic carboxylic acids with polyhydric alcohols, with or without theaddition of hydroxycarboxylic acids, and using starting materials whichcontain imido groups. In place of the free acids and/or alcohols it isalso possible to employ their reactive derivatives. As the carboxylicacid component it is preferred to employ terephthalic acid, whilepolyhydric alcohols which are employed with preference are ethyleneglycol, glycerol and tris(2-hydroxyethyl) isocyanurate (THEIC), thelatter being particularly preferred. The use of tris(2-hydroxyethyl)isocyanurate leads to an increase in the softening temperature of theenamel film obtained.

The imido-containing starting materials can be obtained, for example, byreaction between compounds of which one is required to possess afive-membered, cyclic carboxylic anhydride group and at least onefurther functional group, while the other contains, in addition to aprimary amino group, at least one further functional group. Thesefurther functional groups are, in particular, carboxyl groups orhydroxyl groups, but may alternatively be further primary amino groupsor carboxylic anhydride groups.

Examples of compounds having a cyclic carboxylic anhydride group with afurther functional group are, in particular, pyromellitic dianhydrideand trimellitic anhydride. However, other aromatic carboxylic anhydridesare also suitable, for example the naphthalenetetra-carboxyliccarboxylic dianhydrides, or dianhydrides of tetra-carboxylic acids withtwo benzene rings in the molecule, in which the carboxyl groups are inpositions 3, 3', 4 and 4'.

Examples of compounds having a primary amino group and a furtherfunctional group are, in particular, diprimary diamines, for exampleethylenediamine, tetra-methylenediamine, hexamethylenediamine,nonamethylenediamine and other aliphatic diprimary diamines. Alsosuitable are aromatic diprimary diamines, such as benzidine,diaminodiphenylmethane, diaminodiphenyl ketone, sulphone, sulphoxide,either and thioether, phenylenediamines, tolylenediamines,xylylenediamines, and also diamines with three benzene rings in themolecule, such as α, α'-bis(4-aminophenyl)-p-xylene or1,4-bis(4-aminophenoxy)benzene, and finally cycloaliphatic diamines,such as 4,4'-dicyclohexylmethanediamine. Other amino-containingcompounds with a further functional group which can be used are aminoalcohols, for example monoethanolamine or monopropanolamines, and alsoaminocarboxylic acids, such as glycine, aminopropionic acids,aminocaproic acids or aminobenzoic acids.

The polyesterimide resins are prepared using known transesterificationcatalysts, for example heavy metal salts, such as lead acetate and zincacetate, and also organotitanates, cerium compounds, and organic acids,for example para-toluenesulphonic acid. As cross-linking catalysts inthe curing of the polyesterimides it is possible to use the sametransesterification catalysts, expediently in a proportion of up to 3%by weight based on the binder.

Solvents appropriate for the preparation of the polyesterimide wireenamels are cresolic and non-cresolic organic solvents, for examplecresol, phenol, glycol ethers, for example methylglycol, ethylglycol,isopropylglycol, butylglycol, methyldiglycol, ethyldiglycol andbutyldiglycol; glycol ether esters, for example methylglycol acetate,ethylglycol acetate, butylglycol acetate and 3-methoxy-n-butyl acetate;cyclic carbonates, for example propylene carbonate; cyclic esters, suchas γ-butyrolactone and, for example, dimethylformamide andN-methylpyrrolidone. Furthermore, it is also possible to employ aromaticsolvents, in combination if desired with the abovementioned solvents.Examples of such solvents are xylene, Solventnaphtha, toluene,ethylbenzene, cumene, heavy benzene, various Solvesso® and Shellsol®grades, and Dessol®.

In accordance with the invention it is also possible to use wire enamelscontaining a polyamideimide as binder. The use of such polyamideimidesin wire enamels is known and is described, for example, in U.S. Pat. No.3,554,984, DE-A-24 41 020, DE-A-25 56 523DE-A-12 66 427 and DE-A-19 56512.

The polyamideimides are prepared in a known manner from polycarboxylicacids or their anhydrides in which two carboxyl groups are in a vicinalposition and in which there must be at least one further functionalgroup, and from polyamines having at least one primary amino group whichis capable of forming an imide ring, or from compounds having at least 2isocyanate groups. The polyamideimides can also be obtained by reactingpolyamides, polyisocyanates which contain at least 2 NCO groups, andcyclic dicarboxylic anhydrides which contain at least one further groupwhich can be subjected to reaction by condensation or addition.

Furthermore, it is also possible to prepare the polyamideimides fromdiisocyanates or diamines and dicarboxylic acids, provided one of thecomponents already contains the imide group. For instance, it ispossible in particular first to react a tricarboxylic anhydride with adiprimary diamine to give the corresponding diimidocarboxylic acid,which is then reacted with a diisocyanate to form the polyamideimide.

For the preparation of the polyamideimides, preference is given to theuse of tricarboxylic acids or anhydrides thereof in which 2 carboxylgroups are in a vicinal position. Preference is given to thecorresponding aromatic tricarboxylic anhydrides, for example trimelliticanhydride, naphthalenetricarboxylic anhydrides, bisphenyltricarboxylicanhydrides, and other tricarboxylic acids having 2 benzene rings in themolecule and 2 vicinal carboxyl groups, such as the examples given inDE-A 19 56 512. Very particular preference is given to the employment oftrimellitic anhydride. As amine component it is possible to employ thediprimary diamines already described in connection with thepolyamidocarboxylic acids. The possibility also exists, furthermore, ofemploying aromatic diamines containing a thiadiazole ring, for example2,5-bis(4-aminophenyl)-1,3,4-thiadiazole, 2,5-bis(3-aminophenyl)-3,3,4-thiadiazole,2-(4-aminophenyl)-5-(3-aminophenyl)-1,3,4-thiadiazole, and also mixturesof the various isomers.

Diisocyanates suitable for the preparation of the polyamideimides arealiphatic diisocyanates, such as tetramethylene, hexamethylene,heptamethylene and trimethylhexamethylene diisocyanates; cycloaliphaticdiisocyanates, for example isophorone diisocyanate,ω,ω'-diisocyanato-1,4-dimethylcyclohexane, cyclohexane 1,3-diisocyanate,cyclohexane 1,4-diisocyanate, 1-methylcyclohexane 2,4-diisocyanate anddicyclohexylmethane 4,4'-diisocyanate; aromatic diisocyanates, forexample phenylene, tolylene, naphthylene and xylylene diisocyanates, andalso substituted aromatic systems, for example diphenyl ether, diphenylsulphide, diphenyl sulphone and diphenylmethane diisocyanates; mixedaromatic-aliphatic and aromatic-hydroaromatic diisocyanates, for example4-isocyanatomethylphenyl isocyanate, tetrahydronaphthylene1,5-diisocyanate and hexahydrobenzidine 4,4'-diisocyanate. Preference isgiven to the use of 4,4'-diphenylmehtane diisocyanate, 2,4- and-2,6-tolylene diisocyanate and hexamethylene diisocyanate.

Suitable polyamides are those polyamides which have been obtained bypolycondensation of dicarboxylic acids or derivatives thereof withdiamines or of aminocarboxylic acids and their derivatives, such aslactams.

The following polyamides can be mentioned by way of example:dimethylenesuccinamide, pentamethylenepimelamide, amide,undecanemethylenetridecanedicarboxamide, hexamethyleneadipamide andpolycaproamide. Particular preference is given to hexamethyleneadipamideand polycaproamide.

As crosslinking catalysts in the wire enamels in connection with thecuring of the polyamideimides, it is possible to employ suitable heavymetal salts, for example zinc octoate, cadmium octoate, tetraisopropyltitanate or tetrabutyl titanats, in a quantity of up to 3% by weight,based on the binder.

In accordance with the invention the internal lubricant is preferablycomposed of from 0.1 to 4.5% by weight of polyethylene wax and from 0.1to 2.0% by weight of wetting agent. Very particularly preferred figuresare from 1.0 to 2.0% by weight of polyethylene wax and from 0.2 to 1.2%by weight of wetting agent. The quantities indicated are based in eachcase on the amount of binder in the wire enamel.

The polyethylene waxes which can be employed in accordance with theinvention are commercially obtainable under the name Luwax®. Thesepolyethylene waxes are distinguished by a narrow molecular massdistribution. Furthermore, they enable the controlled establishment ofhigh hardness and high crystallinity.

If a polyethylene wax dispersion, for example Luwax® in xylene, ispoured into an N-methylpyrrolidone-containing solution of theabove-described binders, phase separation takes place. If, on the otherhand, wetting agent is added, then phase separation can be suppressed tovarying degress.

In accordance with the invention, therefore, wetting agents are added tothe wire enamel formulation. The wetting agents advantageously employedare, in particular, fatty alcohol ethoxylates. Emulan® AF, a product ofBASF AG, is particularly suitable for stabilizing the polyethylene waxesdescribed in a wire enamel. Wetting agents which have been fully testedand found suitable also include the BASF products Emulen® EL, Emulan® POand Pluronic® 8100.

The present invention also relates to a process for the preparation ofthe described wire enamel formulation. In this process, solvent is firstof all added to a polyethylene wax having a molecular mass of preferablyfrom 3000 to 6000 [g/mol]. It is preferred to add from 5 to 25% byweight of solvent, based on the polyethylene wax. A proportion ofsolvent of from 8 to 11% by weight is particularly preferred. Veryparticular preference attaches to a figure of 10% by weight. Solventswhich can be employed in particular are aromatic fractions. Xylene andtoluene are preferred above all.

In a further step, polyethylene wax and solvent are heated, preferablyat 70 to 100° C. A temperature of around 80° C. is very particularlypreferred. After the polyethylene wax has dissolved completely, thesolution is cooled to room temperature again.

A wetting agent, preferably fatty alcohol ethoxylate, is then added.

The proportions are chosen such that, preferably, from 0.1 to 4.5% byweight of polyethylene wax and from 0.1 to 2.0% by weight of wettingagent are employed, based in each case on the amount of binder in thewire enamel. These figures are, with very particular preference, from1.0 to 2.2% by weight of polyethylene wax and from 0.2 to 1.2% by weightof wetting agent.

Finally, the dispersion thus obtained is added to the wire enamelobtaining components known per se. Particularly suitable wire enamels inthis context are those whose binders comprise the above-describedpolyesterimides or polyesteramideimides.

The wire enamels according to the invention, prepared in this way, findapplication in particular in the coating of electrical conductors.

In the text below, the invention is described in more detail withreference to examples.

EXAMPLES Example 1

Preparation of a Polyesterimide Wire Enamel

A polyesterimide is prepared by reacting 3.9 parts of ethylene glycol,8.7 parts of dimethyl terephthalate, 10.2 parts of tris(2-hydroxyethyl)isocyanurate, 11.5 parts of trimellitic anhydride and 5.9 parts of4,4'-diaminodiphenylmethane in the presence of 0.04 part oftetra-n-butyl titanate. This polyesterimide is dissolved in 56 parts ofa mixture of aresol/Solventnaphtha° in a ration of 2:1, and 0.7%, basedon the overall formulation, of a commercial titanium catalyst is added.The wire enamel obtained in this way has a viscosity of 800 mPas (23°C.) and a solids content of 39% (1 g/lh/180° C.).

Example 2

Preparation of a Polyamideimide Wire Enamel

A polyamideimide is prepared by the method described in DE-B 12 66 427from 38.5 parts of trimellitic acid and 60.0 parts of diphenylmethanediisocyanate. The wire enamel is a 25% strength solution of thispolyamideimide in a mixture of 65 parts of N-methylpyrrolidone and 35parts of xylene. This wire enamel has a viscosity of 230 mPas at 23° C.

Example 3

Preparation of a Luwax® AH6 dispersion in Xylene

900 g of xylene and 100 g of Luwax® AH6 are heated at 80° C. After thewas has dissolved, the solution is cooled. 20 g of Emulan® AF are addedto the cooled dispersion.

Example 4

Preparation of a Luwax® A dispersion in xylene

900 g of xylene and 100 g of Luwax® A are heated at 80° C. After the waxhas dissolved, the solution is cooled. 20 g of Emulan AF are added tothe cooled dispersion.

Example 5

Preparation of a Polyesterimide Wire Enamel with Internal Lubricant

50 g of the dispersion from Example 3 are added to 1000 g of the wireenamel from Example 1. The enamelling material prepared in this way isapplied.

    ______________________________________                                        Enamelling conditions                                                                             One-coat enamelling                                       ______________________________________                                        Oven:               MAG AW/1A                                                 Temperature:        520° C.                                            Application system: nozzles                                                   Wire diameter:      0.71 mm                                                   Take-off speed:     30 m/min                                                  Number of passes:   10                                                        Degree of increase: 2L                                                        ______________________________________                                    

Example 6

Preparation of a Polyamideimide Wire Enamel with Internal Lubricant

50 g of the dispersion from Example 4 are added to 1000 g of the wireenamel from Example 2. The enamelling material prepared in this way isapplied as a cover enamel over a commercial THEIC polyester base enamel.

    ______________________________________                                        Enamelling conditions                                                                            Two-coat enamelling                                        ______________________________________                                        Oven:              MAG AW/1A                                                  Temperature:       520° C.                                             Application system:                                                                              nozzles                                                    Wire diameter:     0.71 mm                                                    Take-off speed:    30 m/min                                                   Number of passes:                                                             Base enamel        8                                                          Cover enamel       2                                                          Degree of increase:                                                                              2L                                                         ______________________________________                                    

The wires from Example 5 and 6 were each subjected to the followingprocedure: from a section of wire about 750 mm long a twist was preparedas described in IEC 851-5/4.3. A 240 mm section was cut from the twist.This section has 10 windings. The opposite ends of the wires from thetwist are clamped into a Lloyd M30K tearing machine. The force, innewtons, required to tear the twist apart at a speed of 200 m/min ismeasured.

For each enamel, five twists were twists were produced and tested. Atthe same time, the testing was also carried out of five twists of a wirecoated with a conventional paraffinic lubricant.

For a standard wire, a mean force of 2.5 newtons was measured. Thefigure found for the wire from Example 5 was 1.5 newtons and that forthe wire from Example 6 was 1.9 newtons.

We claim:
 1. A wire enamel formulation comprising a binder, at least oneinternal lubricant comprising a polyethylene wax having a molecular massof from 3000 to 6000, and a wetting agent comprising a fatty alcoholethoxylate.
 2. The wire enamel formulation of claim 1, wherein thelubricant comprises from 0.1 to 4.5% by weight of polyethylene wax andfrom 0.1 to 2.0% by weight of wetting agent, the percentages by weightbeing based on the amount of binder in the wire enamel.
 3. The wireenamel formulation of claim 1 further comprising a binder comprising apolyesterimide or polyamideimide.
 4. A process for the preparation ofthe wire enamel formulation of claim 1, the process comprising:a)providing a polyethylene wax having a molecular mass of 3000 to 6000, b)adding from 5 to 25% by weight of a solvent to the polyethylene wax toform a mixture, the percent by weight being based on the weight of thepolyethylene wax, c) heating the mixture to a temperature sufficient todissolve the polyethylene wax to form a solution, d) cooling thesolution, e) adding a wetting agent comprising a fatty alcoholethoxylate to the cooled solution to form a dispersion, and f) addingthe dispersion to a wire enamel comprising a binder so as to produce thewire enamel formulation of claim
 1. 5. The process according to claim 4,further comprising adding from 8 to 11% by weight of solvent to thepolyethylene wax.
 6. The process of claim 4 wherein the solvent isselected from the group consisting of xylene or toluene.
 7. The processof claim 4 wherein heating the mixture to a temperature sufficient todissolve the polyethylene wax to form a solution comprises heating themixture to a temperature of between 70 to 100° C.
 8. The process ofclaim 4 wherein the dispersion comprises from 0.1 to 4.5% by weight ofpolyethylene wax and from 0.1 to 2.0% by weight of wetting agent, basedin each case on the amount of binder in the wire enamel.
 9. The processof claim 4 further comprising adding the dispersion to the wire enamelcomprising a binder selected from the group consisting of apolyesterimide or a polyamideimide.
 10. A process for coating electricalconductors with a wire enamel, the process comprising providing anelectrical conductor, coating the electrical conductor with a wireenamel formulation obtained by the process of claim
 4. 11. The wireenamel formulation of claim 2, wherein the at least one internallubricant comprises from 1.0 to 2.2% by weight of polyethylene wax andfrom 0.2 to 1.2% by weight of wetting agent.
 12. The process of claim 7comprising heating the mixture to a temperature of about 80 degrees C.13. The process of claim 8 wherein the dispersion comprises from 1.0 to2.2% by weight of polyethylene wax and from 0.1 to 2.0% by weight ofwettting agent, based on the amount of binder in the wire enamel.